Here’s more details
The Bottlehead DAC can handle 16,24 and 32 bit audio data at sample rates of 44.1, 48, 88.2, 96, 176.4, 192kHz, and (via USB) 352.8 and 384kHz. Sample rates through 192kHz can be handled by the USB (asynchronous UA2), coax digital and TOSLINK optical inputs (assuming your TOSLINK source is capable of transmitting 192 kHz). No switches need to be thrown to select different sample rates, the sample rate is automatically sensed by the DAC and displayed on a classic 7 segment red LED display inside the 4.5” x 3” x 7.25” (approx.) cabinet. The images shown are of our prototype. We have a little more attractive design in process and we will update the images to show the production model when we have it complete.
The DAC chip used is the Texas Instruments Burr Brown PCM5142 Delta Sigma DAC with 112dB S/N, exceptionally low out of band noise (for reduced EMI and aliasing), integrated negative charge pump line driver (which means no output coupling capacitors, ICs or tubes, and a low output impedance) and intelligent muting (no pops).
As John puts it,
“The FPGA is the “brains” of the whole thing. It is the S/PDIF decoder, buffer controller, output format generator, adjustable oscillator interface, display controller and several others I can’t think of at the moment. The XMOS processor is for the USB interface.”
One of the most significant aspects of the DAC is the oversampling reconstruction filter that John has designed. We actually turn off the filter that comes in the DAC and use John’s instead. Here’s John on filters:
“For those few that haven’t heard my thoughts on digital filters, a quick synopsis: As far as I know all digital filters inside DAC chips use special DSP “tricks” to get really good spec sheet numbers at low cost, BUT these tricks wreak havoc with the “musicality” of the sound. This is why people like NOS DACs. BUT with a NOS you still have the aliases which cause a “dirty” sound. So what I am doing is relaxing the requirements of the filter, so it doesn’t have to meet those insane numbers in the DAC spec sheets, the result is I can implement a simple filter that fits in an inexpensive FPGA.
The result is something most people have never heard, the musicality of NOS without the “dirty” sound of the aliases. It’s simply stunning. Listening to your favorite music with this filter is a whole new experience.
There HAS to be a filter, if you just send a 44.1 or 96 or whatever, the DAC chip will use its own. The only way to turn it off is to upsample to 352 or 384.
BTW a reconstruction filter is not evil; it is a very good thing. It’s just that all the DAC chip implementations have made compromises that adversely affect the sound quality. That is what my filter is all about, doing it right so you get the good things from a proper reconstruction filter without the bad side effects of the DAC chip implementations.
As to buzz words, my own reconstruction filter is an FIR with about 1200 taps. At 44.1 (or 48) input it is an 8X upsampling filter. The FIR is implemented with a single 52 bit MAC running a loop going through the coefficients and intermediate results. This loop runs at 96MHz no matter what the sample rate.
The filter is intermediate phase. A linear phase filter has equal amounts of pre and post ringing but fairly short amount of time in each. The minimum phase has no pre ringing, but in order to get the same over all similar functionality it has to have a much longer amount of post ringing. The intermediate phase splits the difference and has a small amount of pre ringing and post ringing that is a little bit longer than linear phase but much less than minimum phase. This is also deliberately a short filter. That means the amount of time it spends ringing is a very short amount of time compared to other filters. The tradeoff is that the ultimate amount of alias reduction is only about 80db, whereas those in modern DAC chips are going to 130 db or more. This is the tradeoff I mentioned, by not aiming for nearly as much ultimate image rejection I get a filter that sounds much better.
There is no dither involved; it simply is not needed in the DAC.
The overall summary is that not having an oversampling reconstruction filter is NOT the best sound. It may be better than some of the horrible sounding filters used in DAC chips, but a properly done oversampling reconstruction filter sounds way better than either no filter or the ones built into the DAC chips. The secret is to find the RIGHT filter and how to implement it without degrading the sound. I have been working on this for many years and what is in the Bottlehead DAC is the culmination of all that research and listening.”
In order to fully realize the benefits of this filter the DAC needs to be constructed in such a manner that the benefits are not masked by power supply noise, jitter, or unnecessary buffer or gain stages. And so these areas have received special care as well.
“This DAC has 6 ultra low noise regulators. It has ground plane isolation with special high quality isolators on the signals. This does a very good job minimizing the ground plane noise from the digital chips from getting into the analog side of things. This gives a marvelous very “black” background. It also significantly cuts down on the outside world affects such as source component interaction and cable affects. It doesn’t completely eliminate them, but it does significantly reduce these interactions.”
And we have future plans to offer a battery supply in kit form that improves the black background even further.
As for jitter:
“There are two different clock “topologies” for a DAC:
Source is master, DAC is slave
Source is slave, DAC is master
There is another USB mode called adaptive, in which DAC is the slave, but the BH DAC does not use this. SOME other DACs use this mode.
The S/PDIF inputs (coax and optical) just work with the source as master and the DAC as slave. Thus the DAC has to somehow synchronize its clock to the data rate from the source. This is traditionally done with a device called a PLL, which is built in to all the S/PDIF receiver chips. PLLs have much higher jitter than a good fixed frequency clock. The BH DAC does not do it this way. It cleans up the S/PDIF signal, and sends it into an FPGA (field programmable gate array) which does the S/PDIF decoding. But the special part is a digitally controlled ultra low jitter clock. This is almost as good as the best fixed frequency clocks. The FPGA tells this clock to speed up or slow down so it is synchronized to the average data rate of the source.
The result of this is that both S/PDIF and USB produce ultra low jitter to the DAC chip. This combination of ultra low jitter from BOTH S/PDIF and USB doesn’t exist in any other DAC. On other DACs one or the other will be significantly worse than the other input.
I have been listening to all three inputs (S/PDIF coax and TOSLINK and USB) and can tell hardly any difference between them. There are some very slight differences, but they are very small and hard to determine. There is only a very tiny difference in sound with different source components (different computers, OS etc, at least the ones I have on hand) I’ve spent a lot of work trying to decrease these sensitivities and I think I’ve done a pretty good job. Normally I would say that a good asynchronous USB implementation will beat a good S/PDIF implementation, BUT in this case I have spent a huge amount of effort (and I really do mean huge) making an exceptional S/PDIF input. The result is that the SQ from all the inputs should be very similar. The upshot is that if you already have an S/PDIF output from whatever, you can keep on using it with the BH DAC and know that you are not loosing anything compared to one of the other inputs.”
Interestingly when we started this project with John we just assumed that the DAC would have a tube output stage. We even built two early prototypes with tube outputs that used our best technology. But the introduction of the PCM5142 into the mix radically altered our perspective. The negative charge pump line driver in the chip (nope, we don’t really understand how it works either) sounded so good that anything we added after it just colored the sound. So there is NO active output stage, the output from the DAC drives a simple final analog RC filter, which is what mostly determines the output impedance of around 500 ohms.
Another great feature of this DAC is upgradeability. There are lots of interesting ideas brewing for future upgrades:
“The firmware for the FPGA is stored in a special flash memory that is soldered to the board. When new firmware is needed you simply ship your DAC to Bottlehead and we will install the upgrade for you for a modest fee.
I’m designing the USB input to handle 8 channels, but the hardware and firmware to couple multiple DAC boxes together will not be available at first, it’s going to take some time to get that ready.
I HAVE built a really good DSD DAC, and have been comparing the two and I can say that the BH DAC with my custom filter sounds very similar to the really good DSD DAC. I have been coming up with a scheme to add the DSD output DAC to the aux connector, so there MIGHT be a DSD add on board later on.”
So why is this not a kit?
As you may gather from the description, this is not just a DAC chip slapped on a minor variant of a developer’s board made compete with the cheap kits on ebay. There are many complex connections between many chips, regulators and other components on a multilayer board, and firmware must be installed and tested after assembly. Thus the DAC PC board needs to be manufactured in a facility that specializes in that kind of production. That board costs many hundreds of dollars. We got high blood pressure just thinking about the first phone call from an unlucky customer who blew that expensive board due to a wiring error or a slip with the meter probes, and we decided that for everyone’s sanity it made the most sense for us to assemble these DACs in house.
There will be at least one supporting kit in the future, a battery supply. We have already tested the idea and it creates an audible improvement in sonics over both the stock universal wall wart supplied with the DAC and a CLCLC linear supply we built.
And then there is the Nixie tube display that we have been dreaming about. That just might make it as a kit too.
OK so how so how do I get one?
Our initial production run of 50 units has sold out and is now shipping. Check back here for news of future production runs.
Listening TOSLINK with awe. You have hit a home run. Every bit as good as my boutique DAC that cost 4 times as much…I am very impressed with the breadth of the sound stage and the sense of realism when listening to known high quality CDs. I have yet to listen to all source inputs and all sample rates but this DAC is very, very good. -davelang
I’m happy to report that after three days of listening, all sample rates appear to be operating
correctly with my minimalist Linux music server setup. A full review of the Bottlehead DAC is
forthcoming. Teaser alert. It’s something pretty special. Scratch that. It’s freaking awesome! As
some others have reported its very close to analog. The closest I’ve been yet with a digital source.
I knew my first test would be Pandora (free). Its music has always sounded flat and lifeless, even
compared to other internet radio. Now the sound stage has depth, clarity and the music sounds
real. Bass seams to have more punch and the highs have more presence and detail. Overall, a
very noticeable improvement. -RPMac
My first imressions are very positve. I am hearing detail in the music I did not hear with my
previous DAC, especially in the high end. I don’t know how to describe it but music sounds more
“real” than it did with my old DAC. -debk
It sounds great, I will let it burn in and get the gains where they need to be before I give my revue.
But first impression, it sounds like vinyl, without the clicks and pops… -2wo
The BHDAC is cool, very well done; a mamajama box. I understand the design chops needed to
code this type of audio conversion. Well done John! The time frame to execute this project,
factoring in the final result, was well worth it. It’s a breath of fresh air. -elcraigo
People keep asking what it sounds like. It doesn’t really sound like anything at all, if that makes
any sense…It sounds natural, open and free of the fatiguing artifacts that we’re so used to in
digital audio and have learned to live with. Female voices are less edgy, complex Orchestral
pieces hang together and don’t get all congested. And you can relax and listen without the feeling
that something is going to hurt you, if you know what I mean. -2wo
I’d have to echo 2wo’s comment that “it doesn’t sound like anything.” Seems transparent and
effortless to me. -caffeinator
The … has a very analog sound that I love. The BH DAC has a more resolved and deeper bass
sound. It reminds me of when you bought a better turntable as a kid. Everything is just clearer and
more presence better deeper bass. There’s really no comparison. -tsingle999
My BH DAC has been powered up about 30 hours. Setup was easy on a Mac…Sound is
awesome! …Piano, vocals, guitar all really good…Very happy with the dac. 44.1 khz rips
(Leonard Cohen Songs on the Road and Jazz at the Pawnshop) are brilliant. -dw
As a warm up I played Clear by Spirit. I have been listening to this album since 1969 when it
came out. I hear detail like that of a MC cartridge without any of the edginess that the MC
cartridges I can afford have. This CD has all the nuances I remember from the LP. On the title
track the muted trumpets sound like Miles Davis on Kind Of Blue, this is a better recording than I
remember. The bass strings together with the bassoons are distinct. Background details are
there, clear and in the proper perspective. The mid bass is more present than I have heard in my
system before. It is not accentuated but filled in from what I am used to. I wasn’t aware that there
was a lack of mid bass till I heard the BH DAC. Also, the transient response is dramatically
improved. As others have already said, the improved decay is noticeable. Next up was The
Wailin’ Jennys 40 Days again I’m struck with the clarity. The sound is just gorgeous. On
Arlington the violin has no edginess, the sound is completely relaxed. The soundstage is wider
with less curving at the edges. -Grainger49
The BH DAC gets better with time. Even streaming Pandora to it the music takes on a new
dimension. I am very impressed with it, my old DAC doesn’t hold a candle to the BH DAC. I hear
nuances in the music I didn’t hear before, and it is simply a pleasure to listen to. -debk
The more I listen, the more I realize how good this BH DAC is. I am listening only to 44.1 from my
denon 2930 player that was upgraded by … to his signature mod. This dac brings me closer to
what i have been searching for (higher resolution, detail without harshness) than anything I have
heard before. Micro and macro dynamics are stunning from a black background. Billy Joel’s
japanese 20 bit pressing of An Innocent Man is breathless (with air to die for). Great
accomplishment Mr. Swenson, Doc, and all involved. -Johnnycopy
My BH DAC arrived on Friday so I spent the weekend listening through my speaker system and
headphone setup and came to the conclusion that it is simply stunning. Has far exceeded my
expectations, and without a doubt it has been worth waiting for. To date my main DAC has been a
… which has a fairly unique design using a discrete transistor output stage and any reviews I have
read always praised it for being refined and analog sounding. The Bottlehead DAC immediately
stomped all over it, everything about it is improved, lovely tight and dynamic low end, sweet
detailed high end, nothing ever sounds harsh or sibilant or badly recorded. Even low quality MP3
files sound good. The more I have listened the one thing that has impressed me the most is its
ability to separate different sounds into different layers. It’s like listening to multiple sound sources
at once as nothing ever seems to get in the way of each other, truly remarkable imaging and
separation. I know it’s a bit cliché to say I am hearing things I have never heard before in songs I
know well, but it really is a case of listening to things differently as there is so much more depth
and detail to be found. I just love it, thanks Bottlehead! -mcandmar
Received unit today and was able to spend a couple hours listening. Used with Windows 8.1
Business software and install was a snap. I quickly verified it was playing back the higher sample
rates and then began serious listening. I was expecting to like it, but knew in advance it couldn’t
sound better than my $4K DAC. Well, I was wrong… this new Bottlehead DAC is much clearer. I
played the 4 or 5 test passages we all seem to have and immediately was able to hear more of
the backgound chatter, cymbals weren’t tizzy, great air around some voices, etc. After about an
hour of testing, I began to just relax and enjoy the music, even playing pieces that I had not heard
before. Speaking completely subjectively, the overall sound reminded me of what I think of as the
modern sound I hear from some of the better transistor amps…huge sound stage, extremely
clean. Of course there’s the chance that in a week I’ll pine for the honey sound of the other DAC,
but right now I feel as if this is my new reference. -glynnw
My BH DAC arrived last Friday . . . while I was away :'( Home on Monday and I hooked it up and
ran a burn in disc for 24 hours. Sat down on Tuesday night to take a quick listen. Put on Sarah
Jarosz Build Me Up From Bones and ended up listening to the whole album. I had tears in my
eyes and I’ve heard this album many times. …I’d been using an … which at this point in time was
my reference having beat out … that had been greatly enhanced and built by … Both these DACs
are in the mid 2k range so having the BH DAC soundly (no pun intended) trounce the … was a
very satisfying experience. After my brief listen what came to mind is and openness, smoothness,
and rich detail that I had never heard. Softened sibilance but no lack of high frequency
involvement. The sound stage seems slightly compressed but may be more real. More listening
will tell. Thank you, thank you, Bottleheads all for a truly remarkable DAC. I can’t wait to do some
more listening. -Chris Adams